Solid-state image sensing apparatuses are roughly classified into CCD sensors and MOS sensors. The CCD sensor generally has a small-noise advantage but requires high power consumption. The MOS sensor is much lower in power consumption than the CCD sensor, but generally suffers large noise. However, the MOS sensor tends to reduce noise, and is expected to attain performance equal to or higher than that of the CCD sensor in the future.
FIG. 1 is a block diagram showing the schematic arrangement of a conventional MOS sensor. The MOS sensor comprises a sensor array 110 in which many photoelectric conversion elements 101 are two-dimensionally arrayed, a vertical shift register 120 for sequentially selecting (activating) the rows of the sensor array 110, a horizontal shift register 130 for sequentially selecting (activating) vertical transfer switches 131 to sequentially connect the vertical signal lines of the sensor array 110 to a common output line 132, and an amplifier 133 for amplifying a signal which appears on the common output line 132.
In the conventional MOS sensor shown in FIG. 1, the horizontal shift register 130 is constituted by series-connecting many registers. Shift data HD input to a register on the first stage is sequentially shifted to the final stage in accordance with a horizontal shift clock HCLK. The output of each register is connected to the gate of a corresponding vertical transfer switch 131. A plurality of switches 131 are sequentially activated one by one in accordance with the shift of shift data. The vertical shift register 120 operates almost similarly to the horizontal shift register 130 though the cycle of a vertical shift clock VCLK is different from that of the horizontal shift clock.
In the conventional MOS sensor shown in FIG. 1, noise on the power supply line is large along with the operations of the shift registers 120 and 130. This problem becomes serious particularly in the horizontal shift register 130 which performs shift operation at a high speed. The noise problem in the conventional MOS sensor will be explained by exemplifying the horizontal shift register 130.
In the conventional MOS sensor, the shift clock HCLK is supplied to all the registers which constitute the horizontal shift register 130 when signals of respective rows are to be successively read out via the common signal line 132, i.e., when the horizontal shift register 130 is to be operated. In the arrangement shown in FIG. 1, the shift clock HCLK is supplied to all the registers which constitute the horizontal shift register 130. Every time the potential level of the shift clock HCLK changes (i.e., at each HCLK edge), a large current flows through the shift clock line, inducing noise on the power supply line or the like.
In the above shift clock supply method, all the registers which constitute the shift register 130 are simultaneously driven, and power consumption by these registers becomes very large.
This problem also occurs not only in an area sensor in which photoelectric conversion elements are two-dimensionally arrayed, but also in a line sensor in which photoelectric conversion elements are arrayed in a line.